How could the year 2000 be a leap year when 1900 was not?

We all know that February is a funny month -- every four years it has one extra day (February 29) instead of the normal 28 days. When February has 29 days, we call it a leap year.

The year 2000 was a leap year. But 1900 was not. And neither 1800 nor 1700 were leap years. But 1700, 1800, 1900 and 2000 are all divisible by 4, so why aren't they all leap years? And why do we have leap years in the first place?

Let's start with the concept of a year. We define a year to be the amount of time it takes for the Earth to make one complete orbit around the sun. The reason we care about our orbital position around the sun is because of the seasons. In the Northern Hemisphere, we expect summer weather to occur around June, July and August, and winter weather to occur in December, January and February.

A normal year is defined as 365 days. However, if you measure the exact amount of time it takes for the Earth to orbit the sun, the number is actually 365.242199 days (according to Encyclopedia Britannica). By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number. To get even closer to the actual number, every 100 years is not a leap year, but every 400 years is a leap year. That brings the average length of the year to 365.2425 days, which is very close to the actual number.

Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 -- it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was.